The invention relates to an optical component for raising the frequency of a fundamental wave of electromagnetic radiation, which component comprises a non-linear optical medium having a refractive index n.sub.1, in which the frequency is raised, and an optical modulation structure.
The invention also relates to an opto-electronic device for raising the frequency of electromagnetic radiation, comprising such a component as a frequency-raising element.
Raising the frequency is understood to mean both frequency doubling and raising the frequency by a factor unequal to two. Frequency doubling is used to great advantage in apparatuses such as laser printers and scanners and in optical apparatuses for reading and/or writing an optical record carrier, because the information density can thereby be increased. Raising the frequency by a factor unequal to two can be realised by combining radiation of, for example, two radiation sources in which the raised frequency is equal to the sum frequency of the frequencies of the radiation emitted by the radiation sources.
A problem occurring when the frequency of electromagnetic radiation in a non-linear optical medium is raised is that the fundamental wave and the higher harmonic wave generated in the non-linear optical medium propagate at different speeds due to the difference in effective refractive index for the fundamental wave and the frequency-raised wave, in other words, there is wavelength dispersion in the non-linear optical medium. Since higher harmonic radiation propagates at a different speed than the fundamental wave from a position along the direction of propagation in the non-linear optical layer in which a part of the fundamental wave is converted into this higher harmonic radiation, and since frequency conversion is effected at different positions, the different waves of the higher harmonic radiation arriving at a given position along the component have different phases, so that destructive interference will occur between the higher harmonic waves generated at different positions along the component if no further measures are taken after a given distance l.sub.c referred to as the coherence length, resulting in extinction of the frequency-raised radiation. If the rise in frequency is a frequency doubling, the coherence length is given by l.sub.c =4/(.lambda.(n.sub.eff,.omega. -n.sub.eff,2.omega.)), in which .lambda. is the wavelength of the fundamental wave, n.sub.eff,.omega. is the effective refractive index for the fundamental wave and n.sub.eff,2.omega. is the effective refractive index for the frequency-doubled wave.
Extinction of the higher harmonic radiation can be prevented by ensuring that the propagation speeds of the fundamental wave and of the frequency-doubled wave are equal to each other. This solution is known as phase matching. An example of a phase matching method is described in the publication "Non-linear integrated optics" by G. I. Stegeman and C. T. Seaton in J. Appl. Phys. 58(12), 1985 in which phase matching is effected by eliminating the wavelength dispersion by means of modal dispersion.
The present invention relates to an alternative solution, ensuring that conversion into higher harmonic radiation is only effected at given positions along the direction of propagation, such that the waves generated at said positions are substantially in phase with each other. The fundamental wave and the second harmonic wave continue to propagate at different speeds, but the destructiveness of the interference between the different second harmonic waves due to this difference in speed is suppressed. This solution is known as quasi-phase matching. Quasi-phase matching can be realised by introducing a spatial periodical modulation into the linear and/or non-linear optical properties of the non-linear optical material. Such a method is known, inter alia from U.S. Pat. No. 4,865,406. This Patent describes a component of the type described in the opening paragraph whose non-linear, optical radiation-conducting layer comprises a polymer. Periodical modulation of the non-linear optical behaviour is realised by poling the polymer by means of a periodically modulated electric field. In this way poled and oppositely poled domains are created in the polymer, which domains alternate with each other and whose dimensions in the direction of propagation are determined by the coherence length which is required to prevent extinction of the frequency-doubled radiation.
The conversion efficiency of the component described in said Patent is determined by the accuracy with which the periodical modulation of the non-linear optical properties of the non-linear optical component compensates the difference in effective refractive indices for the two waves. Since this difference in refractive indices is not only determined by the properties of the material but also by the thickness of the non-linear optical layer, a drawback of this method is that a relatively small variation in thickness of the non-linear optical layer may give rise to incomplete phase matching resulting in a low efficiency. Notably if the non-linear optical layer is a thin-film layer such as, for example a polymer, it is difficult to maintain its thickness constant throughout the component.